A boiler is a closed vessel where water or other liquid is heated. The fluid will not always boil. (In THE UNITED STATES, the term "furnace" is normally used if the purpose is not to boil the fluid.) The warmed or vaporized liquid exits the boiler for use in various processes or heating system applications,[1][2] including drinking water heating, central heating system, boiler-based power era, cooking food, and sanitation.

Materials
The pressure vessel of the boiler is usually manufactured from steel (or alloy steel), or of wrought iron historically. Stainless steel, of the austenitic types especially, is not used in wetted parts of boilers credited to corrosion and stress corrosion breaking.[3] However, ferritic stainless is often found in superheater sections that will not be exposed to boiling drinking water, and electrically heated stainless shell boilers are allowed under the Western european "Pressure Equipment Directive" for creation of steam for sterilizers and disinfectors.[4]
https://en.wikipedia.org/wiki/Boiler
In live steam models, copper or brass is often used because it is more fabricated in smaller size boilers easily. Historically, copper was often used for fireboxes (especially for vapor locomotives), due to its better formability and higher thermal conductivity; however, in newer times, the high price of copper often makes this an uneconomic choice and cheaper substitutes (such as metal) are used instead.

For a lot of the Victorian "age group of steam", the only materials used for boilermaking was the highest grade of wrought iron, with assembly by rivetting. This iron was often extracted from specialist ironworks, such as at Cleator Moor (UK), observed for the high quality of their rolled plate and its suitability for high-reliability use in critical applications, such as high-pressure boilers. In the 20th century, design practice moved towards the use of metal instead, which is stronger and cheaper, with welded structure, which is quicker and requires less labour. It should be mentioned, however, that wrought iron boilers corrode much slower than their modern-day steel counterparts, and are less vunerable to localized pitting and stress-corrosion. This makes the durability of older wrought-iron boilers much superior to those of welded metal boilers.

Cast iron might be used for the heating vessel of home drinking water heaters. Although such heaters are usually termed "boilers" in a few countries, their purpose will be to produce warm water, not steam, and they also run at low pressure and try to avoid boiling. The brittleness of cast iron helps it be impractical for high-pressure vapor boilers.
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Energy
The foundation of heat for a boiler is combustion of some of several fuels, such as wood, coal, oil, or gas. Electric vapor boilers use level of resistance- or immersion-type heating elements. Nuclear fission is also used as a heat source for generating steam, either directly (BWR) or, in most cases, in specialised high temperature exchangers called "vapor generators" (PWR). Warmth recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbine.

Boiler efficiency
there are two solutions to measure the boiler efficiency 1) direct method 2) indirect method

Immediate method -immediate approach to boiler efficiency test is more usable or more common

boiler efficiency =Q*((Hg-Hf)/q)*(GCV *100 ) Q =Total steam stream Hg= Enthalpy of saturated vapor in k cal/kg Hf =Enthalpy of give food to water in kcal/kg q= quantity of energy use in kg/hr GCV =gross calorific value in kcal/kg like family pet coke (8200 kcal/KG)

indirect method -to measure the boiler efficiency in indirect method, we need a following parameter like

Ultimate analysis of fuel (H2,S2,S,C moisture constraint, ash constraint)
percentage of O2 or CO2 at flue gas
flue gas temperature at outlet
ambient temperature in deg c and humidity of air in kg/kg
GCV of fuel in kcal/kg
ash percentage in combustible fuel
GCV of ash in kcal/kg
Configurations
Boilers can be classified into the following configurations:

Container boiler or Haycock boiler/Haystack boiler: a primitive "kettle" where a open fire heats a partially filled water box from below. 18th century Haycock boilers produced and stored large amounts of very low-pressure vapor generally, hardly above that of the atmosphere often. These could burn wood or most often, coal. Efficiency was very low.
Flued boiler with one or two large flues-an early forerunner or type of fire-tube boiler.

Diagram of the fire-tube boiler
Fire-tube boiler: Here, water partially fills a boiler barrel with a small volume still left above to accommodate the steam (vapor space). This is the kind of boiler used in all steam locomotives nearly. Heat source is in the furnace or firebox that has to be held permanently surrounded by water in order to keep the temperatures of the heating system surface below the boiling point. The furnace can be situated at one end of the fire-tube which lengthens the path of the hot gases, thus augmenting the heating surface which may be further increased by causing the gases reverse direction through a second parallel pipe or a bundle of multiple pipes (two-pass or come back flue boiler); alternatively the gases may be taken along the sides and then beneath the boiler through flues (3-pass boiler). In case there is a locomotive-type boiler, a boiler barrel stretches from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increases the heating system surface in comparison to a single tube and further improves heat transfer. Fire-tube boilers will often have a comparatively low rate of steam creation, but high vapor storage capacity. Fire-tube boilers mostly burn solid fuels, but are readily adaptable to the people of the gas or liquid variety.

Diagram of a water-tube boiler.
Water-tube boiler: In this kind, tubes filled up with drinking water are arranged in the furnace in a true number of possible configurations. Usually the drinking water tubes connect large drums, the lower ones containing water and the top ones water and steam; in other situations, such as a mono-tube boiler, drinking water is circulated by a pump through a succession of coils. This kind provides high steam production rates generally, but less storage space capacity than the above mentioned. Water pipe boilers can be designed to exploit any warmth source and tend to be preferred in high-pressure applications since the high-pressure water/steam is included within small size pipes which can withstand the pressure with a thinner wall structure.
Flash boiler: A flash boiler is a specialized type of water-tube boiler in which tubes are close jointly and drinking water is pumped through them. A flash boiler differs from the type of mono-tube vapor generator where the tube is permanently filled with water. Super fast boiler, the pipe is kept so hot that the water feed is quickly flashed into vapor and superheated. Flash boilers acquired some use in cars in the 19th century and this use continued into the early 20th century. .

1950s design steam locomotive boiler, from a Victorian Railways J class
Fire-tube boiler with Water-tube firebox. Sometimes the two above types have been mixed in the following manner: the firebox includes an set up of water tubes, called thermic siphons. The gases then go through a typical firetube boiler. Water-tube fireboxes were installed in many Hungarian locomotives,[citation needed] but have met with little success far away.
Sectional boiler. Within a ensemble iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside ensemble iron areas.[citation needed] These areas are assembled on site to make the finished boiler.
Safety
See also: Boiler explosion
To define and secure boilers safely, some professional specialized organizations like the American Society of Mechanical Technicians (ASME) develop requirements and regulation rules. For example, the ASME Boiler and Pressure Vessel Code is a typical providing an array of guidelines and directives to ensure compliance of the boilers and other pressure vessels with safety, design and security standards.[5]

Historically, boilers were a way to obtain many serious injuries and property destruction as a consequence to poorly understood engineering principles. Thin and brittle steel shells can rupture, while welded or riveted seams could start poorly, resulting in a violent eruption of the pressurized steam. When water is converted to vapor it expands to over 1,000 times its original quantity and travels down steam pipes at over 100 kilometres per hour. Because of this, vapor is a superb way of moving energy and heat around a site from a central boiler house to where it is necessary, but with no right boiler feed water treatment, a steam-raising plant are affected from range corrosion and formation. At best, this boosts energy costs and can result in poor quality vapor, reduced efficiency, shorter vegetation and unreliable operation. At worst, it can result in catastrophic failing and loss of life. Collapsed or dislodged boiler pipes can also spray scalding-hot steam and smoke from the air intake and firing chute, injuring the firemen who weight the coal into the open fire chamber. Extremely large boilers providing hundreds of horsepower to operate factories can potentially demolish entire buildings.[6]

A boiler which has a loss of feed drinking water and is permitted to boil dry out can be hugely dangerous. If give food to drinking water is then sent into the vacant boiler, the small cascade of inbound drinking water instantly boils on connection with the superheated metal shell and leads to a violent explosion that can't be managed even by safety vapor valves. Draining of the boiler can also happen if a leak occurs in the steam source lines that is bigger than the make-up water source could replace. The Hartford Loop was invented in 1919 by the Hartford Steam Boiler and Insurance Company as a method to help prevent this problem from taking place, and thus reduce their insurance promises.[7][8]

Superheated steam boiler

A superheated boiler on a steam locomotive.
Main article: Superheater
Most boilers produce vapor to be utilized at saturation temp; that is, saturated vapor. Superheated steam boilers vaporize the water and then further warmth the vapor in a superheater. This provides steam at much higher heat range, but can decrease the overall thermal efficiency of the vapor generating flower because the higher steam temperature requires a higher flue gas exhaust temp.[citation needed] There are several ways to circumvent this issue, by providing an economizer that heats the give food to drinking water typically, a combustion air heater in the hot flue gas exhaust route, or both. You will find benefits to superheated vapor that may, and often will, increase overall efficiency of both steam generation and its own utilization: gains in input temperature to a turbine should outweigh any cost in additional boiler problem and expense. There may be useful limitations in using wet steam also, as entrained condensation droplets will damage turbine blades.

Superheated steam presents unique safety concerns because, if any system component fails and allows steam to flee, the high temperature and pressure can cause serious, instantaneous injury to anyone in its path. Since the escaping steam will initially be completely superheated vapor, detection can be difficult, although the intense heat and sound from such a leak clearly indicates its presence.

Superheater procedure is similar to that of the coils on an fresh air conditioning unit, although for a different purpose. The steam piping is directed through the flue gas route in the boiler furnace. The heat range in this area is between 1 typically,300 and 1,600 °C (2,372 and 2,912 °F). Some superheaters are glowing type; that is, they absorb warmth by radiation. Others are convection type, absorbing heat from a liquid. Some are a combination of both types. Through either method, the extreme heat in the flue gas path will heat the superheater steam piping and the steam within also. While the heat of the vapor in the superheater increases, the pressure of the vapor will not and the pressure remains the same as that of the boiler.[9] Almost all steam superheater system designs remove droplets entrained in the steam to avoid damage to the turbine blading and associated piping.

Supercritical steam generator

Boiler for a charged power plant.
Main article: Supercritical steam generator
Supercritical steam generators are frequently used for the production of electric power. They operate at supercritical pressure. As opposed to a "subcritical boiler", a supercritical vapor generator operates at such a higher pressure (over 3,200 psi or 22 MPa) that the physical turbulence that characterizes boiling ceases to occur; the fluid is liquid nor gas but a super-critical fluid neither. There is no generation of steam bubbles within water, because the pressure is above the critical pressure point of which steam bubbles can form. As the fluid expands through the turbine stages, its thermodynamic state drops below the critical point as it can work turning the turbine which turns the electrical generator that power is eventually extracted. The fluid at that time may be considered a mix of steam and liquid droplets as it goes by into the condenser. This results in less fuel use and for that reason less greenhouse gas production slightly. The term "boiler" should not be used for a supercritical pressure vapor generator, as no "boiling" occurs in this product.
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Accessories
Boiler fittings and accessories
Pressuretrols to control the vapor pressure in the boiler. Boilers generally have 2 or 3 3 pressuretrols: a manual-reset pressuretrol, which functions as a basic safety by setting the upper limit of steam pressure, the operating pressuretrol, which settings when the boiler fires to keep up pressure, and for boilers equipped with a modulating burner, a modulating pressuretrol which handles the amount of fire.
Basic safety valve: It is used to relieve pressure and stop possible explosion of a boiler.
Water level indications: They show the operator the level of liquid in the boiler, also known as a sight cup, water measure or water column.
Bottom blowdown valves: They provide a means for removing solid particulates that condense and lie on underneath of a boiler. As the name indicates, this valve is situated straight on underneath of the boiler usually, and is occasionally opened to use the pressure in the boiler to press these particulates out.
Continuous blowdown valve: This enables a small level of water to flee continuously. Its purpose is to avoid water in the boiler becoming saturated with dissolved salts. Saturation would lead to foaming and cause drinking water droplets to be carried over with the steam - a condition known as priming. Blowdown is often used to monitor the chemistry of the boiler water also.
Trycock: a type of valve that is often use to manually check a water level in a container. Most entirely on a drinking water boiler commonly.
Flash container: High-pressure blowdown enters this vessel where the vapor can 'flash' safely and become used in a low-pressure system or be vented to atmosphere as the ambient pressure blowdown moves to drain.
Automatic blowdown/constant heat recovery system: This technique allows the boiler to blowdown only once makeup water is flowing to the boiler, thereby transferring the utmost amount of heat possible from the blowdown to the makeup water. No flash tank is normally needed as the blowdown discharged is close to the heat of the make-up water.
Hand openings: They are metal plates installed in openings in "header" to allow for inspections & installing pipes and inspection of internal surfaces.
Vapor drum internals, a series of display screen, scrubber & cans (cyclone separators).
Low-water cutoff: It really is a mechanical means (usually a float switch) that can be used to turn from the burner or shut down fuel to the boiler to prevent it from jogging once the water moves below a certain point. If a boiler is "dry-fired" (burned without drinking water in it) it can cause rupture or catastrophic failing.
Surface blowdown collection: It provides a way for removing foam or other lightweight non-condensible chemicals that have a tendency to float on top of the water inside the boiler.
Circulating pump: It is designed to circulate drinking water back again to the boiler after it has expelled some of its heat.
Feedwater check valve or clack valve: A non-return stop valve in the feedwater line. This can be fitted to the medial side of the boiler, just below the water level, or to the very best of the boiler.[10]
Top give food to: Within this design for feedwater injection, the water is fed to the top of the boiler. This can reduce boiler exhaustion caused by thermal stress. By spraying the feedwater over a series of trays water is quickly warmed which can reduce limescale.
Desuperheater pipes or bundles: Some pipes or bundles of pipes in the water drum or the steam drum made to cool superheated steam, in order to supply auxiliary equipment that does not need, or may be damaged by, dry out vapor.
Chemical substance injection line: A connection to add chemicals for controlling feedwater pH.
Steam accessories
Main vapor stop valve:
Steam traps:
Main steam stop/check valve: It can be used on multiple boiler installations.
Combustion accessories
Gas oil system:fuel oil heaters
Gas system:
Coal system:
Soot blower
Other essential items
Pressure gauges:
Feed pumps:
Fusible plug:
Inspectors test pressure measure attachment:
Name plate:
Registration plate: